Not so very green

[FCIN]

Ed, a reaction with the layer of green paint is a good explanation. Using less alcohol might be helpful.
How do you mix alcohol and water? I did never find a perfect formula. Too much water and it stands in drops on gloss paint (nasty black spots) and too much alcohol and the paint is dissolved (may ruin fine print and lines). It looks like it depends also on the kind of paint. e.g. dull Tamiya is very sensitive to alcohol while Walthers 40' boxcars have a very glossy but robust surface and need a lot of alcohol or the water stays in drops on the surface.

Lance Mindheim constantly mentions using a mixture of alcohol and india ink to "tone down" everything. I haven't had the nerve to try that on anything, as I often use 91% alcohol to strip paint off models and I'm scared that I'd ruin a perfectly good car/structure if I over did it. Maybe 70% alcohol wouldn't be quite as dangerous as it's already been watered down? But on the other hand, using a method like I mentioned, with a very thinned white, gray or even black paint might be the way to go?

Since my modeling era is the late 70's to early 80's, there weren't that many rust buckets running around and there were many new cars with colorful paint schemes on the rails. Thus, I'm happy with minimal weathering - a "good enough" slightly used effect. You can see the minimal weathering I applied to the two MDC covered hoppers in this photo - more or less the effect you got by accident:     If you've got some old cars to test with, you might try a couple of different methods. As I said, the car without the black wash looks just fine to me.